The present application is directed to AC-DC mains fed power supplies which convert an incoming AC line voltage (which may be nominally 100 Volts to 240 Volts having a frequency of 60 or 50 Hz) to a DC output voltage. The DC output is typically a low voltage of less than 60 volts. Certain applications however provide a higher DC voltage output but require galvanic isolation including for example lighting application. These types of supplies are used to power computers and other electronic equipment and generally provide a power output of less than 500 Watts. The present application is directed at universal power supplies where the supply is designed to work from a nominal 100 Volt to 240 Volt supply at either 50 or 60 Hz. Accordingly, the converters to which the present application is directed are wide input voltage converters intended to operate from an input RMS voltage of 90V to 264V AC at either 50 Hz or 60 Hz. This design constraint is not inconsequential. There is a general push for improved performance across a wide variety of different performance criteria. These criteria include for example efficiency, reliability, Power Factor, EMI, size and cost. Generally there is a balance to be made, for example improving efficiency may increase cost or necessitate a larger EMI filter to meet EMI performance requirements. Thus depending on the particular application and requirements, different types of converter may be employed and a variety of different approaches are known in the art.
For example, a simple form, if a high power factor is not important, comprises a rectifier bridge employed directly to feed a bulk capacitor to which a switching DC-DC converter may be connected. However, apart from a low Power Factor other problems associated with such a design include the general need to limit surge current. Accordingly, the use of such designs tends to be limited to low power arrangements.
For higher powers, there are a number of possibilities, some of which will be discussed below. However generally there are two conventional approaches used in switching AC-DC converters. The first approach uses an isolated flyback converter. The advantage of using an isolated flyback converter is that it has inherently high power factor by virtue of its manner of operation without the need for a specific Power Factor Correction (PFC) circuit. However, it suffers from a number of disadvantages including the need for high voltage switches.
The more conventional approach uses an initial (typically boost) stage as a front end converter which, provides a relatively high DC voltage which is held by a “bulk” electrolytic capacitor. A second stage is employed then to convert the DC voltage to the required lower output voltage. The second stage generally provides isolation in the form of a transformer to isolate the high voltage mains AC side from the lower DC output side.
The required capacitance value is determined by the extent to which the voltage across the bulk, usually an electrolytic, capacitor value is designed to be allowed to drop and this in turn determines the design details associated with the isolation stage. More generally, a designer might start with the design of the isolation stage and work backwards to determine the size of capacitor value required.
It is generally accepted that designing isolation stages for high conversion ratios will involve a compromise in terms of efficiency. The symmetrical LLC converter is widely used and is more immune than most converters in this context, in that it operates close to optimal operating conditions—near the series resonant frequency—under normal operation, and the boosting function (involving lower efficiency) is required only in the few milliseconds where such operation is required. However, whilst the advantage of the LLC converter in these situations is that they are extremely efficient, a disadvantage is that the efficiency is generally at the expense of gain range and their Q factors tend to be relatively low. The net result of which is that they are designed to operate over a relatively narrow input voltage range, which necessitates a reasonably high value of bulk capacitor at their input. There are some known variations on the theme of a boost converter followed by an LLC converter. For example, US20120262954 discloses an arrangement in which a boost converter output is provided to an LLC converter and in which the switching of the LLC converter is provided by the arrangement of the boost converter.
More particularly, in the LLC design, the transformer is typically implemented on a core of low effective permeability to achieve a relatively low value of magnetising inductance. However, designing an LLC converter to operate over a very wide voltage range may involve compromise in terms of requiring excess Q in normal operation, with consequent excessive circulating current and associated losses. So whilst operating over a very wide voltage range may appear to offer an advantage with respect to the reduction of the required capacitor value there are disadvantages to such an approach.
Whilst the PFC stage along with an LLC, flyback or forward-derived isolation stage are the preferred arrangements for AC-DC converters, a variety of other arrangements have been proposed. These alternatives include the isolated boost arrangement. This is basically a combination of a standard boost converter followed by a four-way switched synchronised bridge that works through a transformer to feed energy storage capacitance on the secondary side. Whilst, this configuration offers some advantages, it is generally not employed because of problems associated with start-up, component utilisation and transformer design issues.
Another proposed approach employs LCC resonant converters. Examples of such are described in Schutten et al, “Characteristics of load resonant converters operated in a high power factor mode”, Applied Power Electronics Conference and Exposition, 1991. APEC '91. Conference Proceedings, 1991, Sixth Annual. In this arrangement, a LCC converter is employed as an AC-DC converter. The advantage of the converter is high efficiency. This converter is however not optimised for universal line input. Another approach, Youssef and Jain, “A Novel Single Stage AC-DC Self-Oscillating Series-Parallel Resonant Converter” IEEE Transactions on Power Electronics, Volume: 21, Issue: 6, Pages 1735-1744, 2006 also employs an LCC converter. This arrangement reduces the need for a high voltage electrolytic by placing this on the secondary side of the transformer. Such approaches make use of parallel resonance and can take advantage of the extra control flexibility associated with a full-bridge of switched active devices.
The present application seeks to provide an alternative to the prior art discussed.